The invention relates to a centrifuge and a method for operating a centrifuge for separating solids from solids laden drilling mud.
In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string, which is rotated to bore the borehole through a formation. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry commercial solids i.e. any purposely added solids, such as lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. It is not uncommon to have 30 to 100 m3 of drilling fluid in circulation in a borehole. The drilling mud contains inter alia expensive synthetic oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia the solids to be removed from the drilling mud. This is achieved by processing the drilling mud. The first part of the process is to separate large solids and lost circulation material from the solids laden drilling mud. This is at least partly achieved with one or more a vibratory separators, such as those shale shakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328. The shale shakers may be cascaded in series of stages, such as three stages: a scalping deck having a large mesh screen suitable for removing colloidal material such as clumps of clay; a primary deck having fine mesh screen for removing large particles (but smaller than the colloidal material) which may include lost circulation material; and a secondary deck having a fine screen to remove small particles, mainly drill cuttings. The decks may be arranged in a single basket or in separate baskets and vibrated with a vibratory mechanism.
Further processing equipment such as a centrifuge may be used to further clean the drilling mud of smaller solids. The centrifuge may be used to remove large and medium size solids, although it is particularly suitable for removing small, heavy particles such as “barites” and thickening agents commonly referred to as “bentonites”. These particles are generally too small for a screen in a shale shaker to remove. The resultant drilling mud is returned to the active mud system of the drilling rig.
A mud engineer will analyze the resultant drilling mud and inter alia: dilute the drilling mud if it is too viscous; add more bentonites if the drilling mud is not viscous enough; and add more barites if the drilling mud is not dense enough for recirculation.
It should also be note that a centrifuge may be used without or ahead of the shale shakers or directly after only one or two stage of screening. Furthermore, the centrifuge may be used to clean drilling mud or other fluids on a rig which are not being continuously circulated in the well.
Centrifuges are typically used in any one of three modes of operation:
1. low gravity solids (LGS) removal, in water based mud (WBM) while meeting environmental discharge criteria, and in oil based mud (OBM/NAF) while meeting environmental discharge criteria
2. barite separation, which sometimes requires two centrifuges; and
3. dewatering, simply discharging as many solids as possible.
Typically, Low Speed Decanting is used for Barite removal. Separating factor 500-700, 4-7 micro-meter particle size. Barite is a dense mineral comprising barium sulfate [BaSO4]. Commonly used as a weighting agent for all types of drilling fluids, barites are mined in many areas worldwide and shipped as ore to grinding plants in strategic locations, where API specifies grinding to a particle size of 3 to 74 microns. Pure barium sulfate has a specific gravity of 4.50 g/cm, but drilling-grade barite is expected to have a specific gravity of at least 4.20 g/cm3 to meet API specifications. Contaminants in barite, such as cement, siderite, pyrrhotite, gypsum and anhydrite, can cause problems in certain mud systems and should be evaluated in any quality assurance program for drilling-mud additives.
Typically, Medium Speed Decanting is used with a separating factor 800 for 5-7 micrometer separation.